Subcutaneous structure and blood vessels under skin are barely visible for naked eyes directly. Without any medical instrument, medical doctors can only rely on the external outline of human body and their anatomic knowledge to recognize and locate subcutaneous structures and blood vessels.
The blood vessels, comprising veins and arteries, are hidden below the epidermis and dermis, and in some cases mixed with the subcutaneous fat or are even behind the bones. Images of the blood vessels under the visible light illumination are therefore extremely faint and barely visible for naked eyes. Before puncture, the doctors often try to make the blood vessels more visible by asking the patients to clench their first or flapping the skin above the blood vessel, but hindered by patients' age and the thickness of subcutaneous fat and etc., the visibility of subcutaneous blood vessels is still not satisfying in most cases. Injections relying on the barely visible images of blood vessels frequently results misalignment of the puncture, causing unnecessary pain in patients and delaying optimal time for medical treatment, even triggering other serious side effects. Apart from blood drawing and injection acted on blood vessels directly, acupuncture and other medical surgery etc. all need the blood vessels to be located accurately, so the blood vessels can be avoided or be treated respectively.
In recent years, a technical approach for solving the problem based on near-infrared (NIR) imaging technology has been proposed. This technical approach is based on the fact that the absorption coefficient of hemoglobin for near infrared from 760 nm and 1000 nm is different from other human tissues around the veins, so image contrast is built up. To implement this technical approach, NIR images of veins are acquired in the first step, then the infrared image is digitized and enhanced in contrast and signal to noise ratio by an image processing unit, enhanced image is finally projected back to human skin surface by a visible light projection device. In this technical approach, which has an augmented reality effect in a broad meaning, the doctors and nurses are able to recognize and locate precisely the subcutaneous blood vessels and conduct various medical treatments and operate in real time.
However, the subcutaneous blood vessels are surrounded by subcutaneous fat and muscular tissues, inevitably causing strong scattering to the infrared image. To add more obstacles, wrinkles, scars and hairs on the skin surface all have strong absorption and scattering effects to attenuate and blur the infrared image. These drawbacks become severe when imaging objects are narrow branches of blood vessels and capillaries. This is simply because that less blood volume and therefore less hemoglobin are in the infrared light path, while the light scatterings from surrounding tissues remain the same, resulting in less absorption and faint contrast in the infrared image. Under the influence of scattering light, the image contrast of the blood vessels to the surrounding tissues is often observed in the range of 0.01 to 0.1.
Due to the optical property of subcutaneous soft tissues, the absorption depths of a subcutaneous layer to different light wavebands are different, in the way of a penetrated depth increasing with the wavelength. The visible light waveband from 420 nm in a color of violet to 550 nm most sensitive to eyes can only penetrate 0.6 mm of epidermis layer, while the red light waveband larger than 690 nm can penetrate epidermis and corium layers to irradiate subcutaneous tissues and partial veins. NIR light waveband from 760 nm to 1000 nm, barely visible for naked eyes, can irradiate deeper subcutaneous tissues and fat layer.
When irradiating the skin, scattered light and reflected light from skin surface will cause cross-talk or noise to images of subcutaneous blood vessels. An image only showing deeper subcutaneous layers can be extracted by removing visible light image information from an original image, which is the principle of digital subtraction technology of infrared light image.